14.5.4 Latent heat transfer

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Evaporation (transpiration in plants, perspiration by animals) is obviously a cooling process. Evaporation amounts to a phase change from liquid to vapour, and consumes a large amount of energy, namely 2.45 kJ g–1 at 20°C. Transpiration thus constitutes an important mode of heat loss, and the reverse applies when water vapour condenses onto a leaf as dew or frost (see Section 14.6.1 for liberation of heat when water freezes).

Water evaporating from wet cell walls inside leaves collects as vapour within substomatal cavities, and passes via stomatal pores and the leaf boundary layer to ambient air (Figures 1.1, 1.2).

Water vapour partial pressure inside leaves of known temperature is commonly taken as being equivalent to the saturation vapour pressure of air at that temperature. The water vapour partial pressure outside transpiring leaves is derived from the atmospheric humidity measurement. That difference in partial pressure drives transpiration, while stomatal and boundary-layer resistances constrain transpiration. Latent heat transfer due to transpirational cooling, or more formally ‘heat flux density’ (JTH), is thus a net outcome of transpirational flux (E) times latent heat of vaporisation (λ):

JTH = E × λ (14.5)

For a typical broad leaf (Figure 14.19) with an area of 0.01 m2, on a mild day (air temperature 20°C, leaf temperature 25°C) with a relative humidity of 50% and a moderate wind (0.8 m s–1), resulting in a transpiration rate of 7.2 µg H2O cm–2 s–1, transpirational cooling would account for dissipation of about one-half of the total energy absorbed, and sensible heat exchange for the other half.

Sensible heat exchange and latent heat exchange are both influenced by boundary-layer resistance, and are therefore subject to variation according to leaf shape, size and aero-dynamic conditions. In addition, latent heat exchange will be subject to control by atmospheric humidity, and will be a more significant component of leaf heat budgets on days of low humidity. Accordingly, leaves on well-watered mesophytes benefit from transpirational cooling in balancing their heat budgets, but desert plants have to rely on sensible heat ex-change to a greater extent. Their generally smaller size would diminish boundary-layer resistance and would thus facilitate heat loss via that means. Small size plus surface coatings to enhance reflectivity could offer a selective advantage under dry conditions.